A step for developing an electrostatic latent image means that charged fine grains are so attracted by electrostatic attraction as to adhere to the surface of an electrostatic image carrier and thereby the electrostatic latent image is made visible. The typical dry-processes each capable of performing such a developing step as mentioned above include, for example, a cascade process, a fur brush process, an impression process, a powder-cloud process and so forth. In any one of these processes, however, every toner used therein shall necessarily have a high fluidity. If such toners have a relatively low fluidity, no toner development can be performed smoothly, so that the image quality may be lowered.
The conventionally well-known processes of endowing toners with fluidity include, for example, a process of adding the fine particles of silica into the toners which were made to be hydrophobic, such as described in Japanese Patent O.P.I. Publication No. 47345/1973. The fluidity of toners can be improved by adding the above-mentioned hydrophobic fine silica particles into the toners and, resultingly, an excellet image having a high quality can be obtained. However, when a series of image reproductions are carried out continuously by making use of such toners, there may be arisen a problem that black spots may be produced on an image being reproduced. The phenomenon is taken place in the course of the following processing steps.
In electrophotograhy and so forth, it is necessary to apply a cleaning step of removing toners remaining on an electrostatic image carrier, after carrying out an image transferring step of transferring a toner image formed on an electrostatic image carrier to another medium such a paper sheet. In the above-mentioned cleaning step, a blade cleaning means is normally used so as to make a cleaning efficiency higher and a staining possibility to the surroundings less. When using the toners being added with the above-mentioned hydrophobilc fine silica particles, however, black spots are produced on an image, because spot-shaped residues are produced on an electrostatic image carrier after the toners passed through such a cleaning blade, so that photoconductivity is lowered in the portions of the black spots.
It is an object of the invention to provide electrostatic image developing toners which do not produce such black spots as described above and are, in addition, excellent in fluidity.
As for the electrostatic image carriers generally used in electrophotograhy and so forth, those of the zinc oxide type, selenium type, cadmium sulfide type or organic photoreceptor type are well-known. Among those carriers, the organic photoreceptor type carriers are preferably used, from the general points of view, such as sensitivity, printing resistance, pollution-free property, cost performance and so forth.
The above-mentioned organic photoreceptors are comprised of an organic substance. Therefore, such organic photoreceptors are readily be oxidized by ozone, for example and the surface thereof is also readily be made hydrophilic when some foreign matter adheres thereto and, in addition, the so-called image blur may sometimes be produced by lowering the surface resistance of the photoreceptor in high humidity conditions.
Another object of the invention is to provide electrostatic image developing toners by which any image blurs can be inhibited even when using an organic photoreceptor which is advantageous for a photoreceptor.
Further, in an image developing or transferring step which is to be carried out in electrophotography and so forth, toners shall necessarily be charged sufficiently by a frictional charge so as to display an excellent developability and an excellent image transferability. If the toners are not sufficiently charged, an undesirable phenomenon, that is so-called a non-image-transferred area, is taken place due to the deterioration of developability especially in the conditions such as in the circumstances of high temperature and humidity.
A further object of the invention is to provide electrostatic image developing toners capable of displaying an excellent developability and image transferability even in the conditions of a high temperature and a high humidity.
A still further object of the invention is to provide a method of developing an electrostatic latent image formed on an organic photoreceptor not accompanying any of the above-mentioned disadvantages.